Thixoforming : Semi-solid Metal Processing

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spinel MgAl2O4 and SiAlONs as suitable for steel thixoforming. Several Si3N4-based ceramics, aluminium titanate (Al2TiO5) and numerous mixed oxides are categorized as suitable to a limited extent; SiC, TiN and cordierite are considered unsuitable. Since the first forming trials using ceramic dies and die segments revealed thermal shock acting on the die surface to be critical, corrosion resistance was replaced by thermal shock resistance as a selection criterion for the semi-solid processing of highmeltingalloys.Hencesiliconcarbideandsiliconnitridecamebackintofocusduetotheir excellent strength, thermal shock resistance and fracture toughness, despite their lower ranking in corrosion resistance and wetting angle screenings. Consequently, in recent work on semi-solid processing of steel, silicon nitride was selected exclusively as tool material [7, 32, 70–73]. However, only a few data are available on the actual performance of Si3N4 dies. Beyer [70] observed rupture of externally prestressed, axial-symmetric Si3N4 liners after only a few forming cycles. Failure was attributed to tensile stresses generated by the armouring on the one hand and insufficient thermal shock resistance and fracture toughness on the other. Analysis of die surfaces after application yielded evidence of corrosive attack, with deterioration of the surface quality [74]. Behrens and co-workers [7, 75] used Si3N4 plunger tops for steel thixoforming and regularly observed rupture of the ceramic die parts due to shrinking of the steel part on the ceramic. Neither analysis of the stresses arising in the ceramic die part nor analysis of the die surface after testing was reported. Other workers, although reporting the application ofSi3N4 die parts, indicated that stress resistance was unsatisfactory in steel thixoforming experiments, without giving any details on tool performance [73, 76]. 8.6.2 Ceramic Die Concepts for Steel Thixoforming 8.6 Bulk Ceramic Forming Toolsj283 In a first approach to the selection of ceramic die materials for the semi-solid processing of metals, the tribochemical attack on forming dies is the most critical load, since the service life of metal forming dies is typically limited by the maximum tolerated shape inaccuracy. However, with the envisaged thixoforming of highmelting alloys using ceramic dies, the thermal process loads proved to be decisive for material selection. As discussed in Section 8.1, the upper thermal process boundary is given by the work alloy, whereas the die preheating temperature determining the lower thermal process boundary may be varied. Any change in thermal process conditions unequivocally affects the entire process layout, owing to the pronounced temperature dependence of the liquid phase content and thus the rheological properties of the semi-solid steels. Furthermore, the microstructure of the work pieces and the resulting mechanical properties are affected. A schematic diagram of the interconnection of thermal boundary conditions, process parameters and work piece quality is depicted in Figure 8.39. With reference to the above-mentioned process analysis, bulk ceramic tools for the semi-solid processing of steels were applied following two different die concepts: (i) conventionally heated tools at working temperatures of 1000 C. The characteristics of the different approaches are given in Table 8.10.
284j 8 Tool Technologies for Forming of Semi-solid Metals Table 8.10 Characteristics of bulk ceramic die concepts for steel thixoforming. Die concept Conventionally preheated dies Self-heating dies Forming method Boundary conditions Process-derived High primary material temperature Thixocasting, thixoforging, thixoextrusion Chemical attack by aggressive ferrous liquid phase Tribological attack by solid particles Equipment-derived Die temperature 200–400 C Die temperature >1000 C Rapid solidification of the semi-solid slurry Retarded solidification Short cycle times of the semi-solid slurry Short flow lengths Increased cycle times High forming forces required Long flow paths Low forming forces required Tool demands Long-term temperature stability at 400–500 C High temperature Short-term temperature stability up to primary stability up to 1200–1400 C material temperature Thermal shock resistance Thermal shock resistance required: DT 1000 K required: DT 200–600 K Good corrosion resistance Excellent long-term Excellent wear resistance corrosion resistance Sufficient wear resistance Figure 8.39 Schematic diagram of the interaction between thermal boundary conditions, process parameters and work piece quality.
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Thixoforming Semi-solid Metal Proce
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Further Reading Herlach, D. M. (ed.
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The Editors Prof. Dr. G. Hirt Insti
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VI Contents 1.3.5.2 Other Aluminium
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VIII Contents 4.6.1 Thixocasting 13
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X Contents 7.4.2 Isothermal Non-ste
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XII Contents 9.4.1.3 Simulation Res
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Preface Semi-solid forming of metal
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XVIII List of Contributors Andreas
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XX List of Contributors Alexander S
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2j 1 Semi-solid Forming of Aluminiu
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10j 1 Semi-solid Forming of Alumini
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Table 1.1 Overview of semi-solid pr
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Part One Material Fundamentals of M
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32j 2 Metallurgical Aspects of SSM
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44j 3 Material Aspects of Steel Thi
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Temperature (ºC) 1550 1500 1450 14
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100j 3 Material Aspects of Steel Th
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106j 4 Design of Al and Al-Li Alloy
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5 Thermochemical Simulation of Phas
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here it can be assumed that the dat
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Figure 5.2 Calculated temperature v
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Figure 5.4 Composition profiles of
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It is clear that C has by far the s
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Figure 5.7 The density of X210CrW12
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5.3 Calculations for the Bearing St
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Figure 5.11 Composition profiles of
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Figure 5.14 The density of 100Cr6.
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area which is available for heat tr
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Part Two Modelling the Flow Behavio
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170j 6 Modelling the Flow Behaviour
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7 A Physical and Micromechanical Mo
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3. Macroscopic averages or homogeni
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displacement boundary conditions or
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and liquid are both assumed to be i
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are the strain rate concentration t
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Semi-solid viscosity (Pa s) into cl
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Page 334 and 335: 9 Rheocasting of Aluminium Alloys a
Page 336 and 337: Figure 9.2 Processes for the semi-s
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Page 340 and 341: 9.2 SSM Casting Processesj317 For t
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Table 9.2 Temperature determination
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and are plausible on the left. Furt
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Figure 9.27 Development of the micr
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some places can be observed as smal
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Figure 9.31 Simulation of the metal
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Figure 9.33 So-called Constant Temp
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Figure 9.35 Comparison of the micro
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9.4 Rheoroutej347 If rounding of th
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9.4.1.2 Parameters: Cooling to the
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Figure 9.36 Results of the 2D-MICRE
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homogeneous microstructure and no d
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grain fragment density [mm -1 ] 200
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can be considered for rheocasting o
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Figure 9.44 Microstructure of Zr/Sc
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Figure 9.45 MAGMAsoft simulation: c
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The cooling to the process temperat
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D grain diameter of a circle DGM De
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27 Doppelbauer, M. (2003) Wirtschaf
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10 Thixoforging and Rheoforging of
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1970s [7]. Also, the quality of the
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10.3 Heating and Forming Operations
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With the solution of the Equations
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10.3.1.2 Modelling of Inductive Hea
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material properties have to be cons
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10.3 Heating and Forming Operations
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Figure 10.9 Experimental results, d
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Figure 10.12 Segregation in compone
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10.3.4 Thixoforging of Steel In thi
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10.3.5 Thixojoining of Steel In the
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einforcing element. The possibiliti
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Figure 10.20 The robot controller a
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Figure 10.21 Experimental results w
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Figure 10.24 Design and working pri
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The investigations show that the rh
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Figure 10.31 Scheme of the heat tra
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Table 10.2 Definition of boundary c
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Figure 10.35 Experimental and simul
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References 1 Koesling, D., Tinius,
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steel billets into the semi-solid s
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412j 11 Thixoextrusion The followin
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414j 11 Thixoextrusion channel with
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416j 11 Thixoextrusion parameter co
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418j 11 Thixoextrusion Both concept
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420j 11 Thixoextrusion should be ju
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422j 11 Thixoextrusion Figure 11.8
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424j 11 Thixoextrusion Table 11.4 C
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426j 11 Thixoextrusion Temperature
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428j 11 Thixoextrusion impacts. Ext
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432j 11 Thixoextrusion shell format
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436j 11 Thixoextrusion Table 11.6 P
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440j 11 Thixoextrusion solidificati
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442j 11 Thixoextrusion GPa pressure
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444j Index arbitrary volume element
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446j Index equilibrium tests 141 eq
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448j Index - importance 273 ion flu
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450j Index oxygen-sensitive element
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452j Index - thixoforming 241 semi-
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454j Index - high pressure 131 - sc
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Thixoforming : Semi-solid Metal Processing

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